The present invention generally relates to catalytic converters for reducing the level of pollutants and, more specifically, to a catalytic converter for destroying ozone.
Environmental control systems for aircraft supply pressurized and conditioned air to the aircraft cabin. The temperature, pressure, and relative humidity must be controlled to provide for the comfort of flight crew and passengers within the aircraft. Typically, environmental control systems receive compressed air, such as bleed air from a compressor stage of an aircraft gas turbine engine, expand the compressed air in a cooling turbine, and remove moisture from the compressed air through a water extractor.
Toxic ozone in the compressed air can become an issue when aircraft cruise at altitudes that exceed 20,000 feet. Modern jet aircraft are typically designed for fuel-efficient operation at relatively high altitudes of 25,000 feet or more above sea level. At these altitudes, the ozone content in ambient air is relatively high and, thus, the air supplied to the aircraft environmental control system can contain a substantial amount of ozone. Air containing ozone can cause lung and eye irritation, headaches, fatigue, and breathing discomfort. Because of these dangers, the Federal Aviation Administration (FAA) requires that ozone levels in airplane cabin air be maintained below specified limits.
It is known within the art to utilize catalytic converters to reduce or eliminate ozone in the air supplied to the aircraft cabin. There are a number of desirable characteristics for an ozone destroying catalytic converter of an aircraft. These characteristics include a) high efficiency of ozone conversion at bleed air operating temperature; b) good poison resistance from humidity, sulfur compounds, oil, dust, and the like, which may be present in the compressed air (for long life and minimum system overhaul and maintenance costs); c) light weight to minimize system parasitic load; d) high structural integrity of catalyst support under extreme heat or vibration shock, which may arise during normal flight conditions (also for long life and minimum system overhaul and maintenance costs); and e) high mass transport efficiency with low pressure drop.
Known within the art are ceramic monolith supports which carry a catalyst on a washcoat applied to their surfaces. For example, U.S. Pat. No. 4,405,507 discloses aluminum honeycomb treated with NaOH. U.S. Pat. No. 5,145,822 discloses catalysts attached by an elastic organic adhesive to a metal foil support. U.S. Pat. No. 6,203,771 discloses a catalytic converter with active metals supported directly on an anodized surface layer to remove ozone. However, none disclose the use of the washcoat to destroy ozone.
In some aircraft, the pressurized air supplied to the cockpit and passenger cabins is supplied by dedicated compressors. The air may contain the same amount of ozone as bleed air, but the temperature of this air may be lower than bleed air. At lower temperatures, the ozone removal efficiency of conventional catalytic converters decreases with time. Several formulations have successfully addressed this issue using precious metals and multiple preparation steps. However, this makes producing the catalytic converters expensive.
Moreover, the catalytic converters of the prior art do not utilize the washcoat to destroy ozone. It would be desirable to provide a system and method that uses the support (washcoat) to destroy ozone, rather than merely providing surface area support for a catalyst.
As can be seen, there is a need for an inexpensive catalyst for catalytic converters that destroy ozone. It would also be desirable for such a catalyst to have a low-temperature ozone removal efficiency that has a minimum decay over time, may be easily integrated with existing airplane bleed air systems, has low weight and low pressure drop, and reduces impact on existing environmental control systems and the plane's fuel consumption.